Start with what the model builders publish
The teams building the models know a lot about how to use them, and some of that knowledge is available in the skills they publish. The skill-creator skill is a good example: instructions from people at the model labs who have spent time figuring out what works.
At the time of this post, my advice was to start with those existing skills or have the model ask you questions to build the context, rather than write it all yourself. You still need taste and enough knowledge of the work to judge what comes back.
What I would add now
By August, I was finding it useful to write more of the instructions myself, especially for skills I'd use repeatedly. I like to pretend I'm explaining the work to a new employee. Read the newer note on X.
Originally posted on X. Lightly edited for reading here.